1. Field of the Invention
This invention relates generally to numerically controlled machine tools having rotating spindles and movable slides and more particularly to the control system and methodology for controlling the movement of these slides to cut normal and tapered threads.
2. Brief Description of the Prior Art and Summary of the Invention
Numerically controlled lathes are often used to manufacture objects having screw threads. It is highly desirable to have incorporated in the numerically controlled lathe the ability to cut a thread when the spindle is moving at a non-constant rotational speed. Traditionally thread cutting systems have utilized a high lag type 1 servo-mechanism. Because of its simplicity, this type of servo-mechanism or controller has been used almost exclusively in numerically controlled equipment. As will be discussed below the type 1 servo-mechanism has various deficiencies which become apparent during certain thread cutting procedures.
Threads are typically cut in an object or workpiece by making repeated passes with a cutting tool. To insure that the machine tool cuts the same thread during these repeated passes, the motion command for the tool is synchronized with the turning motion of the spindle. In prior art systems incorporating the type 1 controller, the machine tool often lags behind the motion command. It can be shown that for a type 1 servomechanism, the lag in any axis is proportional to the feedrate along that axis. An expression for the lag along the ith axis is given below: EQU LAG.sub.i =(1/K.sub.v)k.sub.i N
where k.sub.i is a component of the thread pitch, N is the spindle speed, and K.sub.v is the servo velocity constant. As can be seen from the above equation, if the spindle speed varies, the lag changes, the tool position shifts in relation to the workpiece, and the thread is ruined. Well designed machines are capable of holding the spindle speed relatively constant to produce good threads. Even so, it is necessary to allow additional time for the spindle speed to stabilize between speed changes and to provide sufficient time at the start of threading for the machine tool to accelerate to a constant rotational velocity and for the following error to stabilize at a new value.
Quite often the end user of the lathe requires that the threads be rough-cut at high spindle speeds, to minimize the cutting time and then to final-cut, polished or honed at a lower spindle speed. This two-pass thread cutting cannot be performed easily with prior art numerically controlled lathes since the following error is proportional to the rate of spindle rotation. Other applications of numerically controlled lathes require threads to be cut having a non-constant lead. Because the lag is proportional to the thread lead as well as to the spindle speed, the lag will vary during the course of the thread cutting. Thus prior art numerically controlled lathes are not suitable for this type of thread cutting either.
It is, therefore, an object of the present invention to solve the deficiencies noted in the prior art. It is a further object of the invention to be able to accurately control the machine tool permitting repeated cutting of a thread at different or varying spindle speeds. It is further object of the invention to provide a numerical control system for cutting tapered threads. And it is yet another object of this invention to provide a numerical control system that can cut threads with leads that vary linearly with spindle rotation. According to the specific embodiments illustrated in the drawings and discussed in detail below, a numerical control system is used to control the motion of a machine tool, such as a lathe, having a rotating spindle and a cutting tool attached to at least one moveable slide. A controller generates command signals which are synchronized to the rotational motion of the spindle. These command signals may take the form of shaped velocity or acceleration profiles providing a first phase during which one or more slides are accelerated to required velocities and during which time the cutting tool is brought into contact with the rotating workpiece. This procedure is followed by a second phase during which a thread is cut and a final phase during which the slide is decelerated and the cutting tool is retracted from the workpiece.
The machine tool further includes slide means responsive to the input command signals for moving the slides along predetermined coordinate axes. The slide means may comprise motors such as stepping motors for moving the slides and appropriate feedback signal transducers. The control system includes compensation means for causing the slides to follow the command signals in a controlled manner. A feature of the present invention is the incorporation within the control system of a compensation network to force the machine tool to operate as a type 2 servo-mechanism or equivalent thereof permitting the slides to follow the command signals with minimal offset errors during threading. The system may also include means for changing the characteristics of the compensation network after threading to force the appropriate slide to operate as a type 1 servo-mechanism to achieve a rapid retract of the tool from the workpiece.
In one embodiment of the invention an analog compensation network is used, while in another embodiment a digital compensation network is used.
Many other objects, advantages and purposes of the invention will be clear from the following detailed description of the drawings.